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Photoemission from hydrogen-chemisorbed benzonitrile-rubidium ion salt film
Volume
5. number
CHEMICALPHYSICSLETTERS
2
PHOTOEMISSION
FROM
ION
SALT
FILM
M. TSUDA and H. INOKUCHI
The Instifule for Solin’ Slate Physics, The Univevsily of’ Tokyo, Robbonpi, Tokyo, Tltc Department of Pltysics. Scltool of Science and Ettginecring, Waseda Unit.-evsity, Received
8 December
Japan and Tokyo, Japan
1969
The photoelectric response of the benzonitrile-rubidium salt was measured and compared its hydrogen-chcmisorbcd sample. The admission of hudrogcn caused a decrease in quantum higher energy region and an increase of the work function.
The photoemission from metals and semiconductors has been studied extensively by many workers. However, very little has been reported about the phctoemission 01 the aromatic-alkali metal ion radical
salts.
R. Suhrman first showed that the efficiency of the photoelectric emission of an alkali metal is enhanced by the evaporation of a hydrocarbon, naphthalene or anthracene, onto sodium (or potassium)
film
[l].
After
this
work,
a few
studies
of photoemission
from the aromatic-alkali metal ion radical salts have been made in order to clarify the mechanism of the charge-transfer process of the complexes [2-51. Recently, some aromatic alkali metal ion radical salts have been found to have a good catalytic activity for hydrogen-deuterium equilibriation (H2 + D2 = 2HD) [6, ‘71. We showed that this catalytic activity originates from the nature of the ion radical salt in solid phase, but not from that of a free ion or ion pair [8]. How the nature of the solid phase plays a predominant in the catalytic activity is not clear.
March 1970
HYDROGEN-CHEMISORBED
BENZONITRILE-RUBIDIUM H. KAWAMURA,
1
role
with that of yield at a
The photoelectric behaviour of the salt in the solid phase will provide a tool for the study of the catalytic activity. In this note, we present the effect of hydrogen chemisorption on the photoelectric response of the benzonitrile-rubidium ion radical salt. Both the preparation of the ion radical salt and the measurement of the photoemission were carried out in vacua (lo-5 - 10-6 torr) due to the instability of the ion radical saltin air as follows: purified rubidium* was first evaporated on the surface of a glass vessel and then purified benzonitrile was introduced in vacua. The dark red coloured ion radical salt solution was immediatelg obtained. The solution was dropped onto a copper disc cathode of 10 mm diameter (3 of fig. 1) and the excess benzonitrile was completely evaporated (for about 80 hours) from the salt to make a film under vacuum. The copper disc * Rubidium
was prepared from its chloride with calcium metal in an evacuated glass
by reduction tube.
Fig. 1. The schematic diagram of the apparatus for photoemission in vacua. (1) The ion salt solution (2) greaseless tzs (3) copper disc with the film (4) Au evaporated fiIm (5) copper rod (6) tungsten rod and moving apparatus (7) oolLector lead
80
(tungsten
rod)
(8) quartz
window
(9) O-ring.
Volume 5. number 2
CHEMICALPH.YSICSLETTERS
measured down to at least low7 electrons per incident quantum (E, q). In the present work, however, the quantum yield could only be measured down to 10-S E, q. Therefore, the threshold energy was obtained by the following method By analogy with the FLower pIot for metals [Q] the quantum yield of the ion radical salts near
16'
Y
the threshold ventional
Id
10.:
Fig. 2. The spectral distribution of the quantum yield for the fresh film and for the hydrogen-exposed film ----o----.
with the deposited film was moved towards the centre of the glass photocell, the inside of which was coated with a sublimated Au thin film and used
as an electron
was
monochromatised
collector.
Fig.
1 shows
delicate @ass photocell. Incident light from a hydrogen discharge with a Hilger-type
the
lamp spec-
The absolute measurement of the light intensity was made by means of an Eppley circular thermopile, the sensitivity of which was 0.30 UV cmz/wW in vacua. An output potential was applied by a Keithly model 241 regulated voltage supply. The photoemissive current was amplified and observed by a Cary model 32 vibrating-reed electrometer; its spectral response was found, under an applied potential of 5 V, to be enough to find a saturated photoemissive current. The measurement of the photocurrent of the freshly-prepared film was carried out firstly. Then, the film exposed to hydrogen gas was prepared as follows: the salt film was in contact with purified hydrogen gas of 10 torr for 100 minutes and was pumped up to 10-S torr. The photocurrent from the hydrogen-exposed film was measured and compared with that of the fresh one. trometer
Fig.
with a quartz
2 shows
1 hkrch 1970
prism.
the photoelectric
yield
curves
as a function of the incident photon energy. To obtain the threshold energy by the direct extra-
polation method, the quantum yield must be
energy
power
may be subjected
to the con-
law,
where Y is the quantum yield, E, the incident photon energy, E th, the threshold energy. For organic complexes, it is shown empirically that a cube root plot of the quantum yield becomes almost linear as a function of the incident photon energy [lo]. The above method was appLied to our result as shown in fig. 3. Thus the obtained values of E th are 5.01 * 0.02 eV for benzonitrile-rubidium ion radical salt and 4.90 5 0.2 eV for the hydrogen-exposed sample. Similar phenomera were also observed for the hydrogen exposeir benzonitrile-sodium film [Ill. Generally, the formation of the salt reveaied the increase in electronic conductivity and quantum yield of photoemission in comparison with its organic component [3]. The mobile eLectron is assumed to originate from the excess charge which transfers from the dOnOFmolecule to the acceptoi
molecule.
caused energy
a descrease
The admission in quantum
of hydrogen
yield
at a higher
region as illustrated in fig. 2. A similar phenomenon appeared in the behaviour of the electronic conductivity, that is a marked decrease of the conductivity with the chemisorption of hydrogen onto the ion radical salt. The above results suggest that the excess charge of the ion radical
6
tir)(ev) Fig, 3. The pwer-law near the threshold
fit for the photoeIectric
for the fresh film -
for the hydrogen-exposed
and
film ---Q
yield
- - - .
81
Volume 5. number 2
salt participates in forming the chemisorption bond with hydrogen. The quantum yield near the threshold ener,y of the exposed sample increased as compared with the fresh one, and the work function of the salt decreased by 0.11 eV with admittance of hydrogen gas. The cause of the decrease may derive from the change of the surface state of the salt with the chemisorbed hydro-
gen or the change of the electronic state of the
ion radical-hydrogen activated complex. However, precise interpretation for the change in work function is not given now, for it is not clear
whether chemisorbed hydrogen affects only the surface state or the bulk state of the ion radical salt as well. Recently, we found that the catalytic activity of the benzonitrile-alkali metal icm radical salts was greatly influenced by the different donor alkali metals [12]. Now, we are studying the photoelectric responses of four benzonitrile-alkali metal (Na, K, Rb, Cs) ion radical salts and their hydrogen-exposed films.
82
1 March 1970
CHEMICAL PHYSICS LETTERS
REFERENCES [l] R. Surmann, 2. Physik 94 (1935) 742; 111 (1937) [2] ?Inokucbi and Y. Harada, Nature 193 (1963) 477. [3] K. Ogino, S. Iwashima, H. Inokuchi and Y. Harada, Bull. Chem. Sot. Japan 38 (1966) 473. [4] D. R. Kearns and M. Calvin, J. Chem. Phys. 34
(1961) 2026. [S] R. Williams and J. Dresncr, J. Cbsm. Phys. 46
(1967) 2133; A. Many, J. Levinson and I. Teucher. Phys. Rev. Letters 20 (1968) 1161; H.Baessler, N.Riehl and G.Vauble, Mol. Cry’&. 9 (1969)
249.
[6] T. Ilondow, H.Inokuchi and N. Wakayama, J. Chem. Phys. 43 (1965) 376%
H. -hokuchi. N, Wakayama, T. Kondow and Y. Mori,
46 (1967) 837. [7] M. Tsuda, H.InokucN and H.Suzuki, J. Phys. Chem. 73 (1969) 1595. [8] M. Tsuda and H. Inokucbi, to be published. [9] R.H. Fowler, Phys. Rev. 38 (1931) 45. [lo] T. Hirooka, M. Kochi, H. Inokuchi, Y. Harada and J.Aihara, Bull. Chem. Sot. Japan 42 (1969) 1481. [ll] H. Kawamura, M. Tsuda and H. Inokuchi, unpublished. pX] M. Tsuda, to be published.
5. number
CHEMICALPHYSICSLETTERS
2
PHOTOEMISSION
FROM
ION
SALT
FILM
M. TSUDA and H. INOKUCHI
The Instifule for Solin’ Slate Physics, The Univevsily of’ Tokyo, Robbonpi, Tokyo, Tltc Department of Pltysics. Scltool of Science and Ettginecring, Waseda Unit.-evsity, Received
8 December
Japan and Tokyo, Japan
1969
The photoelectric response of the benzonitrile-rubidium salt was measured and compared its hydrogen-chcmisorbcd sample. The admission of hudrogcn caused a decrease in quantum higher energy region and an increase of the work function.
The photoemission from metals and semiconductors has been studied extensively by many workers. However, very little has been reported about the phctoemission 01 the aromatic-alkali metal ion radical
salts.
R. Suhrman first showed that the efficiency of the photoelectric emission of an alkali metal is enhanced by the evaporation of a hydrocarbon, naphthalene or anthracene, onto sodium (or potassium)
film
[l].
After
this
work,
a few
studies
of photoemission
from the aromatic-alkali metal ion radical salts have been made in order to clarify the mechanism of the charge-transfer process of the complexes [2-51. Recently, some aromatic alkali metal ion radical salts have been found to have a good catalytic activity for hydrogen-deuterium equilibriation (H2 + D2 = 2HD) [6, ‘71. We showed that this catalytic activity originates from the nature of the ion radical salt in solid phase, but not from that of a free ion or ion pair [8]. How the nature of the solid phase plays a predominant in the catalytic activity is not clear.
March 1970
HYDROGEN-CHEMISORBED
BENZONITRILE-RUBIDIUM H. KAWAMURA,
1
role
with that of yield at a
The photoelectric behaviour of the salt in the solid phase will provide a tool for the study of the catalytic activity. In this note, we present the effect of hydrogen chemisorption on the photoelectric response of the benzonitrile-rubidium ion radical salt. Both the preparation of the ion radical salt and the measurement of the photoemission were carried out in vacua (lo-5 - 10-6 torr) due to the instability of the ion radical saltin air as follows: purified rubidium* was first evaporated on the surface of a glass vessel and then purified benzonitrile was introduced in vacua. The dark red coloured ion radical salt solution was immediatelg obtained. The solution was dropped onto a copper disc cathode of 10 mm diameter (3 of fig. 1) and the excess benzonitrile was completely evaporated (for about 80 hours) from the salt to make a film under vacuum. The copper disc * Rubidium
was prepared from its chloride with calcium metal in an evacuated glass
by reduction tube.
Fig. 1. The schematic diagram of the apparatus for photoemission in vacua. (1) The ion salt solution (2) greaseless tzs (3) copper disc with the film (4) Au evaporated fiIm (5) copper rod (6) tungsten rod and moving apparatus (7) oolLector lead
80
(tungsten
rod)
(8) quartz
window
(9) O-ring.
Volume 5. number 2
CHEMICALPH.YSICSLETTERS
measured down to at least low7 electrons per incident quantum (E, q). In the present work, however, the quantum yield could only be measured down to 10-S E, q. Therefore, the threshold energy was obtained by the following method By analogy with the FLower pIot for metals [Q] the quantum yield of the ion radical salts near
16'
Y
the threshold ventional
Id
10.:
Fig. 2. The spectral distribution of the quantum yield for the fresh film and for the hydrogen-exposed film ----o----.
with the deposited film was moved towards the centre of the glass photocell, the inside of which was coated with a sublimated Au thin film and used
as an electron
was
monochromatised
collector.
Fig.
1 shows
delicate @ass photocell. Incident light from a hydrogen discharge with a Hilger-type
the
lamp spec-
The absolute measurement of the light intensity was made by means of an Eppley circular thermopile, the sensitivity of which was 0.30 UV cmz/wW in vacua. An output potential was applied by a Keithly model 241 regulated voltage supply. The photoemissive current was amplified and observed by a Cary model 32 vibrating-reed electrometer; its spectral response was found, under an applied potential of 5 V, to be enough to find a saturated photoemissive current. The measurement of the photocurrent of the freshly-prepared film was carried out firstly. Then, the film exposed to hydrogen gas was prepared as follows: the salt film was in contact with purified hydrogen gas of 10 torr for 100 minutes and was pumped up to 10-S torr. The photocurrent from the hydrogen-exposed film was measured and compared with that of the fresh one. trometer
Fig.
with a quartz
2 shows
1 hkrch 1970
prism.
the photoelectric
yield
curves
as a function of the incident photon energy. To obtain the threshold energy by the direct extra-
polation method, the quantum yield must be
energy
power
may be subjected
to the con-
law,
where Y is the quantum yield, E, the incident photon energy, E th, the threshold energy. For organic complexes, it is shown empirically that a cube root plot of the quantum yield becomes almost linear as a function of the incident photon energy [lo]. The above method was appLied to our result as shown in fig. 3. Thus the obtained values of E th are 5.01 * 0.02 eV for benzonitrile-rubidium ion radical salt and 4.90 5 0.2 eV for the hydrogen-exposed sample. Similar phenomera were also observed for the hydrogen exposeir benzonitrile-sodium film [Ill. Generally, the formation of the salt reveaied the increase in electronic conductivity and quantum yield of photoemission in comparison with its organic component [3]. The mobile eLectron is assumed to originate from the excess charge which transfers from the dOnOFmolecule to the acceptoi
molecule.
caused energy
a descrease
The admission in quantum
of hydrogen
yield
at a higher
region as illustrated in fig. 2. A similar phenomenon appeared in the behaviour of the electronic conductivity, that is a marked decrease of the conductivity with the chemisorption of hydrogen onto the ion radical salt. The above results suggest that the excess charge of the ion radical
6
tir)(ev) Fig, 3. The pwer-law near the threshold
fit for the photoeIectric
for the fresh film -
for the hydrogen-exposed
and
film ---Q
yield
- - - .
81
Volume 5. number 2
salt participates in forming the chemisorption bond with hydrogen. The quantum yield near the threshold ener,y of the exposed sample increased as compared with the fresh one, and the work function of the salt decreased by 0.11 eV with admittance of hydrogen gas. The cause of the decrease may derive from the change of the surface state of the salt with the chemisorbed hydro-
gen or the change of the electronic state of the
ion radical-hydrogen activated complex. However, precise interpretation for the change in work function is not given now, for it is not clear
whether chemisorbed hydrogen affects only the surface state or the bulk state of the ion radical salt as well. Recently, we found that the catalytic activity of the benzonitrile-alkali metal icm radical salts was greatly influenced by the different donor alkali metals [12]. Now, we are studying the photoelectric responses of four benzonitrile-alkali metal (Na, K, Rb, Cs) ion radical salts and their hydrogen-exposed films.
82
1 March 1970
CHEMICAL PHYSICS LETTERS
REFERENCES [l] R. Surmann, 2. Physik 94 (1935) 742; 111 (1937) [2] ?Inokucbi and Y. Harada, Nature 193 (1963) 477. [3] K. Ogino, S. Iwashima, H. Inokuchi and Y. Harada, Bull. Chem. Sot. Japan 38 (1966) 473. [4] D. R. Kearns and M. Calvin, J. Chem. Phys. 34
(1961) 2026. [S] R. Williams and J. Dresncr, J. Cbsm. Phys. 46
(1967) 2133; A. Many, J. Levinson and I. Teucher. Phys. Rev. Letters 20 (1968) 1161; H.Baessler, N.Riehl and G.Vauble, Mol. Cry’&. 9 (1969)
249.
[6] T. Ilondow, H.Inokuchi and N. Wakayama, J. Chem. Phys. 43 (1965) 376%
H. -hokuchi. N, Wakayama, T. Kondow and Y. Mori,
46 (1967) 837. [7] M. Tsuda, H.InokucN and H.Suzuki, J. Phys. Chem. 73 (1969) 1595. [8] M. Tsuda and H. Inokucbi, to be published. [9] R.H. Fowler, Phys. Rev. 38 (1931) 45. [lo] T. Hirooka, M. Kochi, H. Inokuchi, Y. Harada and J.Aihara, Bull. Chem. Sot. Japan 42 (1969) 1481. [ll] H. Kawamura, M. Tsuda and H. Inokuchi, unpublished. pX] M. Tsuda, to be published.